After adding a dormer, cut an opening for it through the roof. 
 Video: Create a Dormer 
-  Create the walls and roof elements that make up the dormer. 
 
- Use the Join Roof tool to join the dormer roof to the primary roof. 
  
Note: Do not use the Join Geometry roof tool for this task, or you may encounter an error when creating the dormer opening. 
  
 
  
 
 
- Open a plan view or an elevation view in which you can see the dormer roof and the attached walls. If the roof is extruded, open an elevation view. 
 
- Click 
  
 (Dormer). 
  
 
 
- Highlight the main roof on the building model, and click to select it. 
  
Watch the status bar to be sure that you are highlighting the main roof. 
  
 
  The Pick Roof/Wall Edges tool is active so that you can pick the boundaries that compose the dormer opening. 
  
 
 
- Place the cursor in the drawing area. 
  
Valid boundaries are highlighted. Valid boundaries are a joined roof or its bottom face, a side face of a wall, a bottom face of a floor, an edge of the roof to be cut, or a model line on the face of the roof to be cut. 
  
 
 
- Click 
  
 (Finish Edit Mode).